Coffee machines are known having a wide variety of structures and operating principles.
In particular, espresso coffee machines are known, in which coffee is prepared by a process of percolation of hot water at high pressure.
In all coffee machines, and more particularly in espresso coffee machines, the quality of the obtained coffee depends on a plurality of factors, in part relating to the raw material (the used coffee, its blend, the grinding quality and degree) and in part relating to the performances of the coffee machine itself, with particular reference to the operating pressure profiles and to water temperature during coffee brewing.
Operating pressure and water temperature must have well determined values in order to obtain a high quality espresso coffee, in which all aromas are optimally extracted from coffee powder. Operating pressure should preferably be of about 9 bar, while water temperature should preferably be comprised between 88° and 96° C. depending on the kind of used coffee blend.
In general, in a per se known manner, coffee machines comprise a hot water generator for obtaining hot water at a desired temperature and at least a brewing group for producing coffee. Said hot water generator may consist of a dedicated boiler for the water to be supplied to the brewing group or of a boiler provided with a heat exchanger. The hot water generator is connected at one side to a cold water source, from which it takes cold water and at the other side to the brewing group for supplying hot water to said brewing group. Cold water entering the hot water generator may come from a water tank or directly from the water mains and the coffee machine comprises a feeding pump for feeding cold water to said hot water generator. The hot water generator allows to heat cold water, thus obtaining heated water at a desired temperature, which is transferred to the brewing group, where it comes into contact with the coffee blend during the espresso coffee brewing step.
Preferably, hot water is transferred from the hot water generator to the brewing group not only during the coffee brewing step, but also during stand-by times between a brewing step and the following one. This allows to keep the brewing group at an adequate temperature, preventing it from cooling down to room temperature in case of long stand-by times between a brewing step and the following one.
To this purpose, the coffee machine, besides a hot water supply pipe from the hot water generator to the brewing group, further comprises a hot water return pipe from the brewing group to the hot water generator: hot water continuously flows by natural convection in the circuit formed by the hot water supply pipe and the hot water return pipe, transfers heat to the brewing group for raising the temperature thereof and finally goes back to the hot water generator at a lower temperature. Here, water coming from the return pipe is heated again, thus inducing a variation in its density. This makes the heated water move upwards, towards the hot water supply pipe, thus promoting water circulation between the hot water generator and the brewing group. During these stand-by times between a brewing step and the following one, no additional cold water is fed to the hot water generator.
The continuous flow of hot water in this circuit allows to keep the temperature of the brewing group at an adequate level and for this reason it is called “thermosiphon circuit”.
The provision of such thermosiphon circuit allows a good thermal steadiness during the stand-by steps between a brewing step and the following one, a very quick operating resumption for the following brewing step and a correct brewing temperature as from the very first brewing step.
An example of a known espresso coffee machine of this kind is disclosed in WO 94/000045.
A further example of a known espresso machine of this kind is disclosed in US 2011/0252976.
US 2011/0252976 discloses a coffee machine including a hot water generator, a brewing group for brewing coffee, a cold water supply pipe connecting a cold water source to the hot water generator, a hot water supply pipe connecting the hot water generator to the brewing group and a hot water return pipe connecting the brewing group back to the water generator. The hot water supply pipe is branched, so that hot water coming from the hot water generator flows through a first branch of said hot water supply pipe and then through the brewing group and the coffee powder during the brewing step, while it flows through a second branch of said hot water supply pipe and then through the brewing group (without coming into contact with the coffee powder) and through the hot water return pipe during the stand-by steps between a brewing step and the following one. Thanks to this arrangement, a pre-heating of the brewing group during the stand-by steps between a brewing step and the following one can be achieved.
However, even if the provision of a thermosiphon circuit as disclosed above allows a very quick operating resumption after a stand-by time between a brewing step and the following one, it cannot guarantee a steady water temperature during said brewing steps.
Yet, steadiness in water temperature during the brewing step is an essential parameter for the quality of the obtained coffee.
Moreover, the provision of a thermosiphon circuit as disclosed above cannot allow to accurately and promptly adjust the water temperature during said brewing steps.
On the other hand, in order to obtain a high quality coffee, it is necessary to be able to adjust the water temperature during the brewing step, in particular according to the characteristics of the used raw material (coffee blend, grinding degree, and so on).
The main object of the present invention is therefore to provide a coffee machine which is free from the above-mentioned limitations of prior art and, more particularly, which allows to keep the water temperature steady during the brewing step.
Another object of the present invention is to provide a coffee machine which allows to control and quickly and accurately adjust the water temperature during the brewing step, independently from the water temperature in the thermosiphon circuit, so that the brewing temperature and the temperature of the brewing group during the stand-by times between a brewing step and the following one are independent from each other.
A further object of the present invention is to provide a coffee machine in which, if several brewing groups are provided, the water temperature during the brewing step can be autonomously and independently controlled and adjusted in each brewing group.
These and other objects are achieved by a coffee machine as claimed in the appended claims.